The invention relates to resistive touch screen systems including a resistive touch screen assembly and an ADC (analog-to-digital converter) for digitizing x and y coordinates of touch points at which touch pressure is being applied to the resistive touch screen assembly, and more particularly to circuitry and an automatic self-test method for avoiding the need to provide external circuitry and testing steps to determine if there is adequate connectivity of the resistive touch screen assembly to a touch screen controller chip and also to determine if there is a short circuit in the resistive touch screen assembly.
The closest prior art is believed to include commonly assigned U.S. Pat. No. 6,246,394 entitled “Touch Screen Measurement Circuit and Method” issued Jun. 12, 2001 to Kalthoff et al., incorporated herein by reference. Also, commonly assigned U.S. Pat. No. 6,738,048 entitled “Touch Screen Controller” issued May 18, 2004 to Bernd M. Rundel, also incorporated herein by reference, is indicative of the state of the art.
As shown in “Prior Art” FIG. 1 herein, the '394 patent discloses a touch screen digitizing system which includes a touch screen unit or assembly 30,31 including a first resistive screen 30 with opposed x+ and x− terminals, a second resistive screen 31 with opposed y+ and y− terminals, and an ADC 22 having first and second reference input terminals 35 and 36, respectively. The various terminals of touch screen assembly 30,31 are connected to corresponding terminals of a touch screen controller (TSC) chip 1A including a first switch 19 which is coupled between a first reference voltage (ground) and the x− terminal, and a second switch 18 which is coupled between the x+ terminal and a second reference voltage +VCC for energizing the first resistive screen 30. A third switch 21 is coupled between ground and the y− terminal, and a fourth switch 20 is coupled between the y+ terminal and VCC for energizing the second resistive screen 31. Switching circuitry 5,17 couples an input of the ADC 22 to the y+ terminal while the first resistive screen 30 is energized and the second resistive screen 31 is not energized, and also couples the input to the x+ terminal while the second resistive screen 31 is energized and the first resistive screen 30 is not energized. More specifically, the various terminals of the resistive screens 30 and 31 are connected to the drains of the various corresponding driver transistors 18, 19, 20 and 21. The structure provides continuous calibration of the full-scale analog touch screen output of the full-scale digital output of the ADC 22 irrespective of sharply different variations that may occur in resistances of the switches and resistive screens.
As a practical matter, the above mentioned connections between touch screen assembly 30,31 and the touch screen controller chip 1A have a wide range of connection resistances which may vary from as little as a few ohms to as much as a few megohms, depending upon the condition and reliability of each connection. The wide range of connection resistances typically is caused by solder connection failures or manufacturing defects in connectors. Also, zero-touch-point-pressure short circuits may occur between the resistive screens such as 30 and 31 due to warping caused by material fatigue of one or both resistive screens or manufacturing defects. In the past, testing for adequate touch screen connectivity to the TSC chip 1A and testing for short circuits in the touch screen assembly 30,31 has been performed manually by the user, wherein multiple manual touches at different touch points of the touch screen assembly 30,31 are required to detect failed connections between the touch screen assembly 30,31 and the TSC chip 1A. The foregoing manual testing has resulted in substantial additional cost.
There is an unmet need for a way to conveniently and automatically determine whether or not the touch screen assembly is adequately connected to the driver transistors in the touch screen control chip, despite the wide range of the touch screen panel resistance and the wide range of connection resistance between the resistive screens and the touch screen controller chip.
There also is an unmet need for a way to conveniently and automatically determine whether or not there is a short circuit between resistive screens of the touch screen assembly caused, for example, by warping of one or both of the resistive screens or by manufacturing defects.
There also is an unmet need for a way to avoid the high costs of testing touch screen assemblies in touch screen systems of the prior art.